Posted By
RoePipi
on 2021-03-04
07:59:57
 |  Solving the Tom Thumb spike mystery
Well, I had to inspect what went wrong that produced the infamous spike problem...
(Image from Tom Thumb's "Game Over" section)
For start, it's known that the playfield map is located at $2A1D-$2E04, consisting of 40x25=1000 shapes, as seen in Tom Thumb Editor. Each of these shapes consist of 4x4=16 blocks, which are defined starting from $2E06. (There is a twist in their numbering, not important for now. And yes, $2E05 is presumably redundant.)
I found out with some searches that $09-$0A points to the playfield, and $12-$13 contains the starting address of the 4x4 blocks. Searching for #$2D (which is the triple spikes block, according to block #$40 at $3206-$3215) a routine became suspicious:
. 3D55 LDX #$2D
. 3D57 JSR $1027
. 3D5A LDY #$0A
. 3D5C TXA
. 3D5D STA ($12),Y
. 3D5F TYA
. 3D60 CLC
. 3D61 ADC #$10
. 3D63 TAY
. 3D64 CPY #$0A
. 3D66 BNE $3D5C
This one puts #$2D spikes in certain shapes, at position #$0A, which is line 3, column 3. It's nowhere near random. The problem is, it runs till shape #$0F:
(Cropped image from RoePipi's childhood Plus/4 game data scans)
#$0D would be enough. After putting #$EA to $3D65, that line will look like this:
. 3D64 CPY #$EA
So no spikes will be put on ambiguous positions. I already tested it, so Tom Thumb can be played over again and again 
Whether it was intentional or not, I don't know. Seems like an unfortunate bug.
|
|
Posted By
Csabo
on 2021-03-04
08:30:26
| Re: Solving the Tom Thumb spike mystery
This. This is the kind of Plus/4 relevant detective work I love to see. Well done!
Also... Paging MIK
|
|
Posted By
Chronos
on 2021-03-04
13:48:14
 | Re: Solving the Tom Thumb spike mystery
RESPECT Bro!
|
|
Posted By
Thomas
on 2021-03-04
18:41:03
| Re: Solving the Tom Thumb spike mystery
Could we maybe get a .prg with that fix for the n00bs of us who can't do that fix themselves?
|
|
Posted By
Csabo
on 2021-03-04
20:55:39
| Re: Solving the Tom Thumb spike mystery
Hey Thomas, sent! (Also welcome back... after 6 years )
|
|
Posted By
retroscener
on 2021-03-04
21:16:09
 | Re: Solving the Tom Thumb spike mystery
Oh wow. Finally the fatal bug has been solved. Well done. I would love a working bug free version. I can see new longplays coming soon on my channel, wave 2, wave 3 etc etc 
|
|
Posted By
Luca
on 2021-03-05
03:02:54
 | Re: Solving the Tom Thumb spike mystery
Fantastic achievement and great analysis RoePipi!. I'm not the kind of guy into Tom Thumb, though I'm always ready to move into fixing after eventual requests, so I didn't know about this bug. Yes we need the fixcrack release now, go for it!
|
|
Posted By
Rüdiger
on 2021-03-05
04:17:12
| Re: Solving the Tom Thumb spike mystery
Very good work.
But what I wish for a lot more: that someone uses the editor and creates a completely new maze ...
|
|
Posted By
RoePipi
on 2021-03-05
06:36:51
| Re: Solving the Tom Thumb spike mystery
Csabo, Chronos: Thanks!
retroscener: Thanks! Well, I'm afraid that finishing the maze a second time reveals no more secrets...
Luca: Thanks! Well, I'm not into fumbling with releases, maybe Csabo would do it... He already made a patched version!
Rüdiger: Thanks. Well... Gotta be on it, yeah
|
|
Posted By
retroscener
on 2021-03-05
07:04:09
| Re: Solving the Tom Thumb spike mystery
Well my thought is that the second wave brings in more spike obstacles to the map. so I’m guessing the difficulty increases by more spikes in following waves. Whether anymore would be impassable is another thing though. Would wave 3 be exactly like wave 2 you mean?
|
|
Posted By
RoePipi
on 2021-03-05
12:47:23
| Re: Solving the Tom Thumb spike mystery
Well, I searched the memory for spikes... I also did a quick check in-game, no more surprises... I guess.
Now I'm onto vulnerability...
Update:
Vulnerability, if anyone is interested: >1C67 EA EA EA (while running the game)
Seems like Apos trainer on Tom Thumb page did it better: Tom is vaporized while colliding, thus no color clash :)
|
|
Posted By
Litwr
on 2021-03-05
12:50:59
| Re: Solving the Tom Thumb spike mystery
Fascinating!
|
|
Posted By
Thomas
on 2021-03-06
11:16:54
| Re: Solving the Tom Thumb spike mystery
Thank you so much, Csabo!
Yeah, still lurking the forum... Hoping for XeO3... 
|
|
Posted By
retroscener
on 2021-03-12
06:14:02
| Re: Solving the Tom Thumb spike mystery
What a long awaited release Tom Thumb Deluxe is. I’ve been waiting most of my life for a fixed fatal bug version. Thanks very much RoePipi
Edit: I had a thought. What would be a good idea (if possible) is to also add a small hack giving the option to begin on wave 2. That way we can all immediately enjoy the new more challenging experience. It will take ages to plough through the whole game to finally find out otherwise.
|
|
